Background: asthma is the most common chronic disease facing childhood. Globally, epidemiologic data estimate that nearly 300 million people worldwide suffer from asthma with expectation that this number will grow by more than 100 million by 2025. Aim: the aim of this study was to gain a better knowledge about the magnitude and the association of bronchial asthma with some socio demographic characteristics and obesity in preparatory and secondary school students in Benha city. Methods: Across sectional study was done among students of preparatory and secondary schools (governmental and private) in Benha city from October 2011 to April 2012. Using multi stage systematic random sample. Results: The prevalence of asthma in our study was 8.2%, about 67.3% of them had their first attacks before six years old. Asthma is significantly more in boys than girls with a ratio of 1.7:1 .Egyptian students in governmental schools were more affected with asthma. Obesity measured by BMI were among the risk factors to increase the prevalence of asthma interacting with family history and BMI reveals no relation with the severity of asthma. Conclusion: there is a strong significant association between asthma and obesity in preparatory and secondary school students.